An ebonite beam B is rocked up and down rapidly by a train of mechanism, and moves the cranks FIG.
At first the rolls were driven by workmen by means of cranks, but later they were worked by horses, mules or water-power.
The low-pressure cylinders drive on the leading crank-axle with cranks at right angles, the highpressure cylinders driving on the trailing wheels.
As the cranks are set go apart, there is no dead centre, and the engine is able to start under full load from any point of the stroke.
Circular shifting is regulated by attaching two or more points of the shifting piece to ends of equal and parallel rotating cranks, or by combinations of wheel-work to be afterwards described.
